Related to this, in classical physics it is well understood that Einstein's equations do not have much content without imposing some reasonable conditions on the distribution of energy-momentum (see e.g., [1]}). These are typically called energy conditions and they come in a variety of flavors; a common one is the so-called weak energy condition \(T_{\mu \nu }u^{\mu }u^{\nu }\ge 0\) , for time-like \(u^{\mu }\) , but there are other, such as the strong energy condition, null energy condition, dominant energy condition, and more besides these (see e.g., [2]}). However many of these conditions appear to be violated by quantum field theory quite generically [3]}, and certainly therefore in semi-classical gravity as well (see e.g., [4]}). On its face this appears problematic. First since many trusted results in gravitational physics, as well as black hole physics and thermodynamics, seem to rely on these conditions. But also because there are arguments that violations of these conditions lead to issues with causality (see e.g., [4]}[6]}[7]}). Nevertheless, it is possible that these problems can be tamed somewhat by resorting to alternative energy conditions (see e.g., [8]}[9]}[10]}).
